As is known, modem telecommunications systems permit the transmission of short SMS type messages (Short Message System), which can be written with the telephone keypad and displayed on the telephone's display, at the same time as the voice signal without causing reciprocal interference. This alphanumeric transmission, which until recently was the prerogative of the radio network mobile telephone systems, is now possible using suitable equipment connected to the fixed network, such as telephones, fax machines, electronic post boxes (E-mail), etc.
The SMS messages are made up of different fields. In the header of the message, there is a field that contains the user's telephone number and is called the OADC field (Originator Address Destination Code), and in the body of the message, there are service and text fields. The user's telephone number permits the system manager to bill the call, and the recipient to identify the origin of the message. This feature, however, although necessary for the manager, may not be acceptable to the sender of the message or may, in fact, clash with the standards governing communication privacy, when for example the SMS messages are used for particular services, such as voting, prize-winning competitions, etc. The supplier of the final service must not be able to obtain the user's personal data via the telephone number, but must be able to reach the user by other means.
The methods currently employed to protect the identity of the caller using a voice telephone are not suitable for this purpose, as they normally work by making the last three figures of the telephone number illegible. This prevents user identification, but it also makes it impossible for the person receiving the call to call back.
Also known, from WO 2000/076231 of Ville Jarvinen, is a method for transmitting a message in a communications service operating through a mobile telephone network, permitting anonymous communications between system users. The telephone number of the sender of each message is replaced by the SMS center with a virtual ID, and such correspondence is stored in a database. The user who receives the message, without being aware of the real telephone number; is able to reply using the virtual ID, as the SMS center is able to associate the virtual ID to the correct number, consulting the database. This solution implies structural changes in the mobile network, for the implementation of the database and for its cooperation with the SMS center. Moreover the size of the database is not negligible and increases with the number of users.
Document EP 0 899 918 (equivalent U.S. Pat. No. 6,591,291) discloses a system for generating an alias source address in a different technical field, i.e. for an electronic mail (e-mail) message. The system includes an alias source address substitutor that substitutes the alias source address for the real source address, using a secret key. This removes the real source address from the e-mail message and thereby renders the sender; located at the real source address, anonymous. Still, a reply e-mail can be sent by means of a remailer, e.g. by adoption of symmetric encryption algorithms. The procedure known from this document is, however; not transferrable to SMS messages, due to the specific structure of the SMS.
These difficulties are avoided and the aforesaid technical problems resolved by using the procedure for transmitting SMS messages with protected identity (the subject of this invention), which prevents the recipient of an SMS message from seeing the caller's telephone number, and therefore the corresponding personal data, but which allows him to contact the caller by sending an SMS message.